The present invention relates to a demodulator for phase difference modulated signals in the microwave frequency range, of the type including phase detectors each having one input connected to directly receive the phase difference modulated signal and its other input connected to receive the phase difference modulated signal after it has been conducted through a time delay element and a phase shifter.
Such a demodulator, in the special case for a four-phase difference modulated signal, is disclosed, for example, in the articles by W. H. Childs et al, "An Integrated DQPSK Demodulator for 14 GHz Satellite Communications Applications," IEEE International Microwave Symposium, Ottawa, Canada, 1978, and by T. Shimura et al, "120-Mbit/s, 6 GHz On-Board Waveform Regenerator for Communications," Proceedings of the 9th European Microwave Conference, Brighton, England, 1979.
The basic circuit of a prior art demodulator for a four-phase difference modulated signal (DQPSK) is shown in FIG. 1. The input signal is here divided and delayed in a delay element VG in one branch by one symbol duration T, corresponding to the bit length of the digital signals. The delayed and the undelayed signals are compared with the aid of two phase detectors PD1 and PD2. The delayed signal is fed to the two phase detectors via respective phase shifters .rho., which produce a total phase difference of .pi./2 between the two delayed signal components applied to the phase detectors. At the outputs of lowpass filters TP1 and TP2 connected behind the phase detectors, there then appear output voltages which, if the phase setting is correct, correspond to the original modulation signal.
In the construction of the circuit shown in FIG. 1, the delay element VG generally is considered to be a particularly critical component because the delay time T must be absolutely constant. The requirements placed on the delay element become the stricter, the greater is the ratio of signal frequency to bit rate. For a signal frequency of 14.25 GHz and a bit rate of 120 Mbit/s, as for example, in satellite data transmissions, a relative change in the delay time by 10.sup.-5, for example, produces a phase error of almost one degree. It is therefore necessary to employ special measures to effect temperature compensation.
The delay elements described in the applicable literature are either lines, as described in the article by Y. S. Lee and W. H. Childs, "Temperature Compensated BaTi.sub.4 O.sub.9 Microstrip Delay Line"., IEEE International Microwave Symposium, Orlando, Fla., 1979, or in the above-cited article by T. Shimura et al, 120 Mbit/s, 6 GHz On-board Waveform Regenerator for Communications," Proceedings 9th European Microwave Conference, Brighton, England, 1979, or bandpass filters in microstripline form, as described in the article by Y. S. Lee, "14 GHz MIC 16-ns Delay Filter for Differentially Coherent QPSK Regenerative Repeater", IEEE International Microwave Symposium, Ottawa, Canada, 1978. Although these delay elements have a relatively good temperature stability, they also exhibit a rather high degree of attenuation, which in the frequency range of 14 GHz is greater than 20 dB. In order to operate the phase detectors at the necessary signal level, the power of the input signal must therefore be increased. Due to the fact that the delayed signal has a lower power level than the undelayed signal because of the high attenuation of the delay element, there will occur shifts in the zero crossings of the detector characteristic which can be compensated only for one input level at a time.
In order to prevent phase errors due to noncompensated shifts when there are fluctuations in amplitude, it is necessary to connect amplitude limiters ahead of the known demodulators. Amplitude limiters cause a degradation of the demodulator performance which can be compensated only by an increase in the output power of the transmitted signal or by a reduction of the noise figure of the amplifier connected ahead of the demodulator.
If the phase detectors of the known demodulators are to be operated with the same amplitude, the signal distribution ahead of the delay member must be nonuniform. This requires higher input level.